It's more than that, if the review comes back into review phase, the comments are still marked read. For instance, if I can quickly see that all the comments are just "Defect marked fixed" comments from other reviewers, I can mark them read. Then if someone makes a "real" comment, I don't have to sift through the ones I previously chose to ignore and ignore them again.

That's an open issue at this point. For now, you start a numbered conversation and state what you are talking about. This is much the same as we do for other binary file types now (e.g., word documents)

The concept is to graphically show the differences between the files. We have this feature in our 5.0 dev code and it works by displaying the before and after images on top of one another and varying the alpha channel of the top image. You can toggle between "before" and "after" or use a slider to see the two blended together. It's useful for icons and other graphical artifacts.